Certain medical implants and orthopedic implants require strength for weight bearing purposes and porosity to encourage bone/tissue in-growth. For example, many orthopedic implants include porous sections that provide a scaffold structure to encourage bone in-growth during healing and a weight bearing section intended to render the patient ambulatory more quickly. Rapid manufacturing technologies (RMT), particularly direct metal fabrication (DMF), or direct metal laser sintering (DMLS), and solid free-form fabrication (SFF), have been used to produce metal foam used in medical implants or portions of medical implants. These technologies are also referred to as additive manufacturing technologies. In general, RMT methods allow for structures to be built from 3-D CAD models, including tessellated/triangulated solids and smooth solids. For example, DMF techniques produce three-dimensional structures one layer at a time from a powder which is solidified by irradiating a layer of the powder with an energy source such as a laser or an electron beam. The powder is fused, melted or sintered, by the application of the energy source, which is directed in raster-scan fashion to selected portions of the powder layer. After fusing a pattern in one powder layer, an additional layer of powder is dispensed, and the process is repeated with fusion taking place between the layers, until the desired structure is complete.
FIG. 1 is a scanning electron microscope (SEM) image of an exemplary porous metal structure built by RMT taken at 50× magnification. As can be seen, such porous metal bone ingrowth structures built by rapid manufacturing techniques comprise fully-molten struts covered with spherical metal micro-particles or beads that are semi-fused or partially fused to the structures. This is because when the laser strikes the powder, it creates a melt pool in which the powder is melted into liquid form and melded to the adjoining area. However, at the very edge of the melt pool, some of the powder particles do not completely melt into liquid form. As a result, after cooling, the surface of the porous structures often contain residual powder particles that are only partially attached to the structure.
When the porous structure is used in a medical implant, these semi-fused micro-particles have the benefit of increasing surface area of the porous structure for cellular attachment and subsequent bone ingrowth in vivo. If these particles are too loosely bound, however, they can detach during implantation or use (e.g., due to micro-motion between the bone and porous structure) and migrate to the joint space, possibly acting as a third body particle and increasing wear of the implant bearing surfaces. Typical post-manufacturing processes, such as machining or polishing, to finish the surface and remove the attached powder particles are not available for porous structures due to the porosity nature of these structures where internal struts or surfaces are out of reach. Furthermore, these post-manufacturing processes pose a contamination concern because machine oil and reagents used in such processes may adversely affect the bone-ingrowth if not adequately removed.
Known post-manufacturing processes to address these micro-particles include removal of the micro-particles by chemical etching, such as that disclosed in U.S. Application Publication No. 2006/0147332, or melting them into the main body of the porous structure, such as that disclosed by Stamp et al (J Master Sci: Mater. Med (2009) 20:1839-48). While these methods ensure that the micro-particles do not detach and harm the patient during use, they also diminish the benefits provided by these micro-particles by reducing the surface area and the asperities of the porous structure for cellular attachment.
In light of the above, there is still a need for efficient methods to improve the attachment strength of the micro-particles without substantially reducing the surface area, asperities, and/or friction of the porous structure for certain benefits, such as cellular attachment and initial fixation.